The present invention relates to materials science, and more specifically, to tip-enhanced Raman microscopy.
Raman spectroscopy is a technique for characterizing the chemical composition of a material sample by illuminating the sample with a laser beam and detecting a shift in the wavelength of the light scattered from the sample. Raman spectroscopy makes use of the physical phenomenon known as “Raman scattering” or “inelastic scattering”, in which the incident light interacts with the rovibronic (rotational-vibrational) modes of molecules without altering the energy states of electrons in the molecules. Raman scattering is a relatively weak phenomenon compared to fluorescence, in which the electron energy states are altered. Because Raman scattering is relatively weak, it is difficult to detect in small samples.
Tip-enhanced Raman microscopy (TERM) is an advanced optical technique in which sample material is deposited on a surface and an atomically small metal probe (e.g., the tip of a scanning probe microscope (SPM) such as a scanning-tunneling microscope (STM) or atomic force microscope (AFM)) is brought close to the surface in conjunction with the focal point of the laser light. The frequency of the incident laser light is selected to be resonant with the plasma frequency of the metal probe, so that the probe tip functions as a plasmonic nanoantenna; thereby, the intensity of the incident laser light and of the scattered light of the sample material is amplified (for relatively small wavelength shifts). This makes it easier to detect the Raman scattered light, which enables characterizing the chemical composition of relatively small samples (potentially as small as single molecules).